A load pump is a device normally used in automatic control phase loops and is more specifically placed between the phase comparator and the loop filter. The automatic control phase loop or PLL (“phase locked loop”) in fact denotes an assembly formed of an automatic controlled phase oscillator and the associated phase comparator and filter.
The phase automatic controlled oscillator is provided to provide a signal whose phase is triggered to that of a reference signal. This concerns a voltage-controlled oscillator whose output signal is applied to a phase comparator. The error voltage, possibly provided by said comparator, is filtered by a lowpass filter so as to eliminate its possible fluctuations and applied to the variable element of the oscillator so as to cancel the phase difference.
Therefore, depending on the signals provided by the phase comparator, the role of the load pump is to inject or remove a constant current from the loop filter. In order to achieve this, a load pump is constituted by a block introducing a current source of the current mirror type and more particularly a first current source is found to generate a negative current so that the outgoing current is controlled according to a symmetrical system. The first source is provided to inject a constant current into the loop filter and the second source is provided to remove a constant current from the loop filter according to the signals provided by the phase comparator. Each source then needs to be constant regardless of the voltage of the loop filter. Thus, this concerns here of being able to provide a constant current whose value is equal to a reference value over the largest possible output voltage range. Now in special operating conditions, a problem is likely to appear. Thus, when the output voltage of the load pump approaches the mains voltage of the load pump (respectively from the ground), the transistor forming the current source which generates a positive current (respectively which generates a negative current) enters a linear state which falsifies the current recopies and the output current is no longer kept at the reference value.
One solution of this problem given by the prior art and described in the European patent application EP 0 778 510 consists of using cascoded current sources. This patent document relates to highly symmetrical bi-directional current sources able to generate equal negative and positive currents over a wide output voltage range. The positive and negative currents generated are moreover rendered equal to a reference current with great precision. This device of the prior art includes a first and a second cascoded current source, each source being coupled to current mirrors so as to apply a constant reference current, respectively a positive and negative reference current, from a minimum output voltage up to a maximum output voltage between the ground of the mains voltage.
Thus, this solution proposes using current sources having a high internal impedance. In fact, an increase of the internal impedance of the current source makes it possible to render the current supplied to be more insensitive to voltage variations at the terminals of the device.
This improvement of the output impedance of the device able to reach the objective in question, namely the application of a constant reference current, either positive or negative, is thus obtained by means of using cascoded current sources.
However, the use of cascoded sources requires working with greater mains voltages with respect to the use of single sources. These cascoded sources in fact make use of two transistors instead of a single one and, according to the document of the prior art, the minimum mains voltage to be applied is three volts. Thus, this requirement of the prior art prevents functioning with small mains voltages. In fact, in this case, a low mains voltage less than said mains voltage would result in a bad recopy of the currents since it is then not possible to place the transistors in a good working condition, namely the saturated state which is characterised by a sufficiently high drain source voltage.
Other solutions not implementing cascoded current sources are also currently known. However, in these solutions, adapting the current source between the positive current source and the negative current source is not taken into account. This phenomenon concerning adapting the current between the positive source and the negative source thus generates significant interference in the output frequency of the oscillator in controlled operational mode.